Sex Determination (లింగ నిర్దారణ) Sri Ranjani Tallam III Sem, Genetics Dept of Zoology, Dodla Kousalyamma Govt College for Women(A), Nellore.

Sex Determination Definition : A sex determination system is a biological system that determines the development of sexual characteristics in an organism

Sex Determination Mechanisms Unisexual (ఎక లింగ జీవులు): Male and Female reproductive organs are present in separate individuals. Bisexual (ఉభయ లింగ జీవులు) : Male and female reproductive structures are present in one organism

History (చరిత్ర) Aristotle (335 B.C.): Sex is determined by “the heat of the male partner during intercourse” Until 20th century: The environment – temperature and nutrients, in particular –was believed to be important Factors favoring the storage of energy and nutrients predisposed one to have female offspring, whereas factors favoring the utilization of energy and nutrients influenced one to have male offspring (Geddes and Thomson, 1890) 8th BC Homer: Conception is influenced by the wind, north (ఉత్తరము) for males and south (దక్శిణ) for females…at least in sheep

German biologist H. Henking discovered the X body in Pyrrhocoris, fire bug. In the later investigations this X body is designated as X chromosome. Y- Chromosome was first identified by Stevens and Wilson in 1905 in meal worm, Tenebrio molitor In male all the chromosomes are paired, but the chromosome homologous to X chromosome is distinctly small. This smaller chromosome is called the Y- chromosome.

Sex determination (లింగ నిర్దారణ) The underlying mechanism by which an individual develops into a female or a male is called sex determination. For most species of animals and some species of plants, sexual reproduction is carried out by individuals of the opposite sex—females and males.

Sex is a character. It has two alternatives, namely maleness and femaleness. The male produces the sperm and the female produces the egg. Sex is determined by the following factors: Chromosomes (క్రొమొసొములు) Extra chromosomal factors a. Male Haploidy (పురుశ ఏక స్థితికము) b. Environment (పర్యావరణం) c. Hormone (హార్మొనులు) d. Metabolism( జీవక్రియలు) f. Parasites (పరాన్నజీవులు)

1. Chromosomal Sex-determination (క్రొమొసొముల ద్వారా లింగ నిర్దారణ) Chromosomes were first described by Strausberger in 1875. The term “Chromosome”, however was first used by Waldeyer in 1888. In most of the animals a pair of chromosomes is responsible for the determination of sex. This pair of chromosomes is called sex chromosomes or Allosomes (లైంగిక క్రొమొసొములు). The chromosomes other than the sex chromosomes are called Autosomes (శాఖీయ క్రొమొసొములు).

If the two sex chromosomes are similar the individual is described as homogametic(సమ సంయొగ బీజదము) , if the two sex chromosomes are different or it contains one sex chromosome, individual is described as heterogametic (విశమ సంయొగ బీజదము).

a. Heterogametic sex determination(విశమ సంయొగ బీజద లింగ నిర్దారణ) Male heterogametic and female homogametic type: XX-Xo method: (Hemiptera and orthoptera Grasshoppers)- McClung

XX-XY type: (Drosophila, Man and Some other mammals):

b. Male homogametic and female heterogametic sex determination . ZZ-ZO type: (Butterflies, Moths and Domestic Chickens):

ZZ – ZW type: (Fishes, Reptiles, Birds and some Insects)

A. Male Haploidy (పురుశ ఏక స్థితికము) 2. Extra chromosomal factors A. Male Haploidy (పురుశ ఏక స్థితికము) In insects belonging to the order Hymenoptera (Apis mellifera (honeybees,ants and wasps), sex is determined by the number of sets of chromosomes (haploid or diploid) a bee receives. Female that feeds on royal jelly becomes the fertile female called queen and others feed on honey become the sterile females called workers. The unfertilized eggs undergo Parthenogenesis, which is an asexual process of embryonic development, to produce haploid fertile males called drones. This Parthenogenetic development of haploid males from the unfertilized eggs is called Arrhenotoky (అనిశేక జననము). Males produce sperms by mitosis rather than meiosis.

B. Environmental control of sex determination (లింగ నిర్దారణ లొ పర్యావరణం పాత్ర) EX: (Rotifers, Annelids, Arthropods, Echiuroids Bonellia viridis-green spoon worm In Bonelia virids, a marine worm, all the larvae are genetically and cytologically similar. If a particular larva settles near the proboscis of an adult female, it becomes a male individual. • On the other hand, if it develops free in water, it becomes a female. The secretions of proboscis in the female influence the young worms to develop in to males.

3. Hormonal effect on sex determination (లింగ నిర్దారణలొ హార్మొనుల ప్రభావము) Crocking Hen: Crew had reported a case of complete sex reversal in fowls. A fertile hen may change to fertile cock due to the damage to the ovary or after the natural cessation of egg-laying. The ovary in the reproductive female secretes a male suppressing hormone. Hence male development is suppressed as long as ovary is active. Testis may develop in the absence of the ovary or in the natural cessation of ovary. • Male hormones are produced earlier. These male hormones circulate to the female foetus during the development suppressing the development of ovary in the female. So, the female co-twin becomes sterile.

FREE MARTINS are known to have been described by the Roman writer  Varro, who called them "taura“. The 18th-century Physician John Hunter  discovered that a freemartin always has a male and female twin. Freemartin: In cattle when fraternal twins, or dizygotic twins (one male and the other female) are produced the female becomes sterile and the male becomes normal. This sterile female co-twin is called freemartin. It was hypothesized early in the 20th century that masculinizing factors travel from the male twin to the female twin through the vascular connections of the placenta because of the vascular fusion, and thereby affect the internal anatomy of the female.

In Rotifer increased oxygen consumption induces the production of more males.

E. Determination of sex by the parasite (లింగ నిర్దారణలొ పర్రాన్న జీవుల పాత్ర) Crab Carcinus maenas, Sacculina If a crustacean parasite called Sacculina is present in males it is changed in to a female in course of time. It may be due to the physiological disturbances caused by the parasite.